Migrate Metric threshold rule tests to deployment agnostic solution

[mgiota]

Part of #183263

🍒 Summary

As part of this PR we introduced a way to write deployment agnostic tests using the deployment agnostic framework from Appex QA team. We want to migrate the existing metric threshold rule tests to the new agnostic framework.

Existing tests

Test	Type	ESS	Serverless	Notes
Metric threshold rule type	Alerting	✅	❌	ESS: x-pack/test/alerting_api_integration/observability/metric_threshold_rule.ts

File structure

How to migrate

In this PR oblt.stateful.config.ts and oblt.serverless.config.ts were added, so no need to create any config file.
Existing metric threshold rule tests use a few alerting helpers. In the SLO burn rate rule PR the alerting API was migrated to the deployment agnostic folder. As part of the migration we should use the migrating alerting api.

Here's what you have to do:

• Move metric threshold rule tests from x-pack/test/alerting_api_integration/observability/metric_threshold_rule.ts within x-pack/test/api_integration/deployment_agnostic/apis/observability/alerting folder
• Add the metric threshold rule test in this file
  loadTestFile(require.resolve('./metric_threshold_rule'));
• Delete the metric threshold rule test from the old alerting_api_integration location
• Use the DeploymentAgnosticFtrProviderContext
• use a combination of supertestWithoutAuth and samlAuth to generate an API key for user roles and make API calls

const samlAuth = getService('samlAuth');
const supertestWithoutAuth = getService('supertestWithoutAuth');

• Use the config service to decide on the expected consumer

const config = getService('config');
const isServerless = config.get('serverless');
const expectedConsumer = isServerless ? 'observability' : 'infrastructure';

• Update the metric threshold rule tests to use the deployment agnostic alerting api

How to run

To run serverless

node scripts/functional_tests_server --config x-pack/test/api_integration/deployment_agnostic/configs/serverless/oblt.serverless.config.ts
node scripts/functional_test_runner --config x-pack/test/api_integration/deployment_agnostic/configs/serverless/oblt.serverless.config.ts --grep="Metric threshold rule"

To run stateful

node scripts/functional_tests_server --config x-pack/test/api_integration/deployment_agnostic/configs/stateful/oblt.stateful.config.ts
node scripts/functional_test_runner --config x-pack/test/api_integration/deployment_agnostic/configs/stateful/oblt.stateful.config.ts --grep="Metric threshold rule"

[elasticmachine]

Pinging @elastic/obs-ux-management-team (Team:obs-ux-management)

[maryam-saeidi]

Moved to backlog until the related Meta ticket is prioritized.

[jasonrhodes]

I think we need to update these instructions so they are up to date with the latest on deployment agnostic testing before this issue is "Ready", but then we need to pull this in ASAP.

[maryam-saeidi]

Example PR: #187924

[mgiota]

It turns out Metric threshold rule type is not registered on serverless, so I am closing this issue. No need to migrate Metric threshold rule types to the deployment agnostic framework.